Laser measuring instrumentation has been used for some time, particularly where probes would interfere with accurate measurement. Such laser devices can include velocimeters, interferometers, and particle sizing instruments. Lasers are used as the light source in these optical measuring devices.
The well-known arithmetic formulas used to calculate measurements taken by laser optical measuring devices generally take into account the laser wavelength. A constant laser wavelength is assumed. However, in certain lasers, notably semiconductor diode lasers, the laser wavelength is subject to undesirable variation. In the case of semiconductor diode lasers, it is believed that the wavelength changes result from temperature and current changes, and laser aging. The changes can occur continuously or in discrete jumps called "mode hops."This laser wavelength variation limits the precision attainable using laser optical measuring devices. Consequently, efforts have been made to stabilize the laser wavelength.
In the past, such efforts have been directed to stabilization of the temperature and current of the semiconductor diode laser, since it is known that temperature and current changes can cause variation in the laser wavelength. However, when implemented with the required level of accuracy, this procedure is expensive, and its complexity reduces overall system reliability. In addition, some wavelength variation is due to laser aging effects, and stabilization of temperature and current does nothing to overcome this aspect of the problem.
Accordingly, a need exists for more accurate laser optical measuring instruments that can produce accurate measurements in spite of laser wavelength variations. Similarly, a need exists for methods of enhancing the accuracy of measurements taken with laser optical measuring instruments, given the variable nature of laser beam wavelength.